The essential elements of the Open Systems Interconnection model OSI Layer 2, e.g. Ethernet, service between IEDs in different substations can be realized over a packet switched network using pseudo wires. A Pseudo Wire PW is a bidirectional entity which offers the mechanism to emulate the essential attributes of a native service over a Packet-Switched Network PSN.
RFC 3985 of the Internet Engineering Task Force IETF describes the architecture for pseudo wire emulation between any two customer edges CEs. The native communication between one customer edge and another customer edge over a PSN is enabled via provider edges PEs. The provider edge devices offer one or more pseudo wires for their customer edges to communicate over the PSN. The provider edges are connected via PSN tunnels that are identified by their PSN labels. A tunnel can carry one or more pseudo wires. The pseudo wires in the tunnel are identified by their pseudo wire labels. The payload that is transmitted from a sender CE is received at the receiver CE in the same form as transmitted by the sender CE i.e. the PSN is transparent to the customer edges.
RFC 5603 of the IETF describes a model for managing Ethernet pseudo wires over a PSN. The model defines port based pseudo wire and VLAN based pseudo wire, with or without manipulation, i.e. changing, adding or removing, of VLAN IDs. The port based pseudo wire connects two physical Ethernet end points while the VLAN based pseudo wire connects two virtual Ethernet end points. RFC 5602 of the IETF describes a model for managing pseudo wire services over MPLS tunnels. A pseudo wire is formed by establishing two Label Switch Paths LSPs, which are essentially unidirectional tunnels.
The Layer 2 traffic in substations, such as time-critical GOOSE and SV messages according to IEC 61850, is typically segmented using VLANs that ensure that the traffic reaches only the subscribed IEDs. This implies that the pseudo wires to other substations have to be configured based on these VLAN IDs. However, it should be noted that the VLAN ID used for Layer 2 traffic could be in conflict at PE because the other client CEs attached to the PE may use same VLAN ID for transporting a different traffic. This implies that the VLAN ID should be translated at egress in the sender substation or an outer 802.1Q VLAN tag should be added. Conversely, the modified VLAN ID should be translated again to the original, or a different VLAN ID at ingress into the receiver substation or the outer VLAN tag should be removed.
IEC 61850-90-1 technical report provides an overview of the different aspects that need to be considered for information exchange between substations. The report also proposes two approaches for inter-substation communication namely tunneling and gateway. The former approach encapsulates IEC 61850 messages for transport over PSN tunnels. The “tunneling” approach encapsulates the message for transport over WAN but does not modify the payload within the message. The “proxy gateway” approach uses a proxy to re-create messages that arc sent by the devices in other substations via WAN. It also translates the content of the message in accordance with the transport protocol used in WAN. In other words, the “proxy gateway” approach does a protocol conversion between IEC 61850 and the PSN transport protocol. Apparently, the gateway should be fully aware of the content of the IEC 61850 messages.
The tunneling approach for GOOSE and SV exchange can be realized using Layer 2 Virtual Private Networks VPNs over MPLS backbone. The MPLS protocol may be IP/MPLS or MPLS-TP. The pseudo wires are the basis for transport over such packet switched network. Pseudo wire is a bidirectional entity that offers the mechanism for emulating a native service over a packet switched network such that it is transparent to the nodes that use the native service. Different connectivity types such as point to point, point to multipoint, or multipoint to multipoint can be realized using pseudo wires. The pseudo wires can transport multiplexed, i.e. VLAN based, or non-multiplexed, i.e. port based, service over MPLS backbone. Different pseudo wire redundancy mechanisms, e.g. 1+1 backup, can be used to provide fault tolerance. The pseudo wires terminate at the edge devices of service providers. The substation WAN edge devices connect to the WAN via the provider edge devices.
However, the conventional substations are not envisioned for inter-substation communication. This implies that the configurations for the substations are derived considering only information and communication model that are applicable within those substations. The extension of substation configuration for inter-substation communication should deal with potential naming and addressing conflicts. This is because IEC 61850 recommends unique values for system wide identification/resolution of certain naming and address parameters, e.g. LDName, VLAN, Multicast address, APPID, goID, svID etc. However, in many applications such as protection, multiple substations are engineered in the same way, where the naming and addressing parameters of IEC 61850 information model in a substation may match with that of the IEC 61850 information model in other substations.
The conflicts of naming and address can be resolved by renaming the message parameters. But this will effect a reconfiguration in some of the devices, that do not take part in “direct coordination” with devices in remote substations or that are not necessarily involved in the WAN communication, in already energized substations. The problem becomes more complex when the substation communicates with more than one substation. This is because the likelihood of naming and addressing conflicts becomes even higher. IEC 61850-90-1 proposes engineering of IEC 61850 communication among substations for tunneling approach based on SED file exchange, with the presumption of a “Greenfield” scenario, where conflicts can be resolved by granting of dataflow engineering rights to other substation projects. However, the standard does not address the naming and/or addressing conflicts that can occur when extending already commissioned substations for inter-substation communication.
In summary, in order to solve the naming and addressing conflicts, most of the utilities will be disinclined to reconfigure the devices in their energized substations. The reconfiguration of the devices may cause interruption of the substation operation.